check_fds.c

/* Copyright (C) 2000, 2002, 2003, 2005 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with the GNU C Library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA. */#include <errno.h>#include <fcntl.h>#include <paths.h>#include <unistd.h>#include <sys/stat.h>#include <sys/sysmacros.h>/* Try to get a machine dependent instruction which will make the program crash. This is used in case everything else fails. */#include <abort-instr.h>#ifndef ABORT_INSTRUCTION/* No such instruction is available. */# define ABORT_INSTRUCTION#endif#include <device-nrs.h>#include <not-cancel.h>/* Should other OSes (e.g., Hurd) have different versions which can be written in a better way? */staticvoid
check_one_fd (int fd, int mode)
{
/* Note that fcntl() with this parameter is not a cancellation point. */if (__builtin_expect (__libc_fcntl (fd, F_GETFD), 0) == -1
&& errno == EBADF)
{
constchar *name;
dev_t dev;
/* For writable descriptors we use /dev/full. */if ((mode & O_ACCMODE) == O_WRONLY)
{
name = _PATH_DEV "full";
dev = makedev (DEV_FULL_MAJOR, DEV_FULL_MINOR);
}
else
{
name = _PATH_DEVNULL;
dev = makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR);
}
/* Something is wrong with this descriptor, it's probably not opened. Open /dev/null so that the SUID program we are about to start does not accidently use this descriptor. */int nullfd = open_not_cancel (name, mode, 0);
/* We are very paranoid here. With all means we try to ensure that we are actually opening the /dev/null device and nothing else. Note that the following code assumes that STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO are the three lowest file decsriptor numbers, in this order. */struct stat64 st;
if (__builtin_expect (nullfd != fd, 0)
|| __builtin_expect (__fxstat64 (_STAT_VER, fd, &st), 0) != 0
|| __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
|| st.st_rdev != dev)
/* We cannot even give an error message here since it would run into the same problems. */while (1)
/* Try for ever and ever. */
ABORT_INSTRUCTION;
}
}
void
__libc_check_standard_fds (void)
{
/* This is really paranoid but some people actually are. If /dev/null should happen to be a symlink to somewhere else and not the device commonly known as "/dev/null" we bail out. We can detect this with the O_NOFOLLOW flag for open() but only on some system. */#ifndef O_NOFOLLOW# define O_NOFOLLOW 0#endif/* Check all three standard file descriptors. */
check_one_fd (STDIN_FILENO, O_WRONLY | O_NOFOLLOW);
check_one_fd (STDOUT_FILENO, O_RDONLY | O_NOFOLLOW);
check_one_fd (STDERR_FILENO, O_RDONLY | O_NOFOLLOW);
}